I have the 20 and consciously chose it over the 25, although I could easily afford either one. But that's me and I can certainly see why others might think differently. The pros and cons are in my opinion as follows:

Pros for the 20

Smaller

Lighter

Less expensive

As sharp or sharper at really wide apertures

At least as good bokeh if by bokeh you mean the quality rather than the quantity of the blur

Wider FoV makes it more useful in close quarters

Reportedly doesn't cause as much aperture clicking ("rattle-snaking")

Pros for the 25

Better low-light ability (about half a stop)

Can provide more background blur by means of its longer FL and greater max aperture

More narrow FoV gives it more reach and makes it possible to take images at close distance/high magnification without too much distortion

Noisefree AF (may be important for video where the noise is recorded if you AF during the clip; not otherwise)

Suitable for AF-C (the 20 is not)

Is not reported to cause lock-ups with the E-M5 (the 20 is, although I have never experienced this myself)

Is not reported to cause banding with the E-M5 (the 20 is, and I have seen it myself; but see further info below)

There has been much talk about the slow AF of the 20. I have tested that myself and found only a very marginal difference in ordinary AF-S compared to lenses that are known to be at least as fast (perhaps faster) as the 25. See here:

I have both and think this list of pros for each lens explains the reason why I keep and use both.

I suspect the 20mm is slightly wider than its nominal focal length because it's very close to my 9-18mm at 18mm. The FOV difference between the 20mm and 25mm is actually pretty different. Here's an illustration of 9-18mm at 18mm, 20mm, and 25mm: